"This all came very suddenly this morning, around 7am local time. Most schools open around that time, classes usually start an hour later," he said.

Employers have been asked to be understanding when their employees say they need to pick their children up from school.

In New York, however, officials said a similar threat to schools had been received and was being treated as a hoax.

"There is no credible threat to our children. We are convinced that our schools are safe," Mayor Bill de Blasio told a press conference, adding that the threat was "so generic and outlandish" that it could not be taken seriously.

"The immediate assessment by the intelligence division, and again, in consultation with the FBI, is that there was nothing credible about the threat," he said.

In Los Angeles, police and FBI agents were called in to help search the more than 1000 schools in the district, Cortines said, adding that he expected the operation to be completed by the end of the day.

Addressing a news conference, New York Police commissioner Bill Bratton suggested Los Angeles school officials had been over-cautious in deciding the shutdown.

"We cannot allow ourselves to raise levels of fear," Bratton said. "Certainly levels of awareness, but it is not a credible threat and not one that requires any action on our part similar to what my understanding is the school system in Los Angeles took."

The chief of the Los Angeles school police department, Steven Zipperman, stressed the decision was an extreme precautionary measure.

"Earlier this morning we did receive an electronic threat that mentions the safety of our schools," he told reporters.

"In an abundance of caution, as the superintendent has indicated, we have chosen to close our schools today until we can be absolutely sure that our campuses are safe."

Zipperman said private schools in the district had remained open since the threat was only directed at Los Angeles’ public school system.

The news conference was held shortly after 7am local time, before the start of the school day for most children.

The Los Angeles Unified School District is the second largest education district in the US, with more than 1000 schools. The district comprises more than 21,000 buildings spread over 1800 square kilometres within Los Angeles and nearby communities.

California was shocked by the massacre of 14 people in San Bernardino earlier this month, in an attack carried out by US-born Syed Farook and his Pakistani wife Tashfeen Malik.

Authorities believe the Muslim couple had been radicalised for some time and may have been inspired by the extremist group Islamic State.